Medios

The apparent absence of PAH features toward the majority of low-mass embedded class I sources could have a number of explanations.

First, PAH molecules are primarily excited by UV photons. The precise shape and strength of the radiation field inside an embedded object is not well-known but in the class I sources the central source has already formed and is the main energy source inside the envelope. The presence or absence of excess UV will influence the PAH emission features. In particular, the high opacity of the envelope at UV and optical wavelengths, which provide the main excitation of PAHs, will constrain the region where PAHs can be excited to a small radius.

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velope close to the star, the dust in the surrounding envelope may provide too high extinction in the optical and mid-infrared for the (PAH) emission to escape, especially if the envelope mass is (still) relatively high compared to the disk. If an outflow cavity is present, the inclination at which the object is observed will affect the extinction by the envelope.

Third, the abundance of small PAH molecules in the gas phase may be significantly lower due to freeze-out due to the low temperatures and high densities in the interior of the molecular core.

In practice, a combination of the above three causes will apply simultaneously. To estimate their effects, models are run, varying several parameters including radiation field, PAH abundance and the mass of the envelope.

Luminosity of the central source and presence of UV excess

Models with total stellar luminosity L∗ varying between 1, 3 and 6 L are shown in

Fig. 5.5. In addition, a model without the template Draine field for UV excess is shown for L∗ = 6 L. Increasing the luminosity by a factor 6 increases the line flux of the

PAH features while the PAH feature / continuum ratio is reduced by at most 20%. Excluding the excess UV field, while preserving the total stellar luminosity, decreases the PAH feature/continuum ratios by ∼3. Interestingly, the 3.3µmfeature is affected most because it requires higher energies to be excited than the other features. For typical luminosities observed toward low-mass embedded protostars, PAH features should be detectable, even if no UV excess is present.

Mass of the envelope

Models with the mass of the envelope Menv varying from 0.1, 0.5, 1.0 to 1.5 M are

shown in Fig. 5.6, at i = 45◦. In all models the PAH features are clearly present. In- creasing the envelope mass by a factor 15 results in a decrease of the emission of the central source while the sub-mm emission and the strength of the absorption features increase. The PAH features decrease in peak flux by a factor 3-4, but are in no case extinguished by the continuous extinction or silicate absorption features. This is con- sistent with the conclusions of Manske & Henning (1999) for higher mass YSO’s.

In the template models, PAHs are located in both the envelope and the disk. To test the influence of the envelope further, model setups including PAHs only in the disk or only in the envelope were performed. A comparison is shown in Fig. 5.7, for Menv = 1.0 and 0.1 M. Including PAHs only in the disk results in a decrease of peak flux

of the PAH features by a factor ∼2. In the SED, the absence of PAHs in the envelope leads to less absorption of UV emission, which appears stronger. The mass of the envelope has no significant effect on the strength of the PAH features, although the underlying continuum changes. Including PAHs only in the envelope also results in a typical decrease of the PAH feature peak flux, by about a factor 1.5. Even if the PAHs are located only in the envelope, PAHs should be detectable if they are present at ISM abundance.

Figure 5.5: Model SED (top) and blow-up of spectrum (bottom) ati= 45◦ for template model parameters andL∗ varying between 6, 3, and 1 L(black, red, blue respectively), all including

UV excess. A model withL∗ = 6L without UV excess is shown in purple. Major PAH and

absorption features are indicated. The inset shows a blow-up of the 3.3µmfeature on the red wing of the water absorption band.

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Figure 5.6: Model SED (top) and blow-up of spectrum (bottom) with Menv = 1.5(black), 1.0

(red), 0.5 (blue) and 0.1 (purple) Mati = 45◦. The curve for 1.0M was omitted from the

Figure 5.7: Model SED (top) and blow-up of spectrum (bottom) with PAHs in both envelope and disk (black), only in the disk (red) and only in the envelope (blue), ati = 45◦. The inset shows a blow-up of the 3.3µmPAH feature.

Outflow cavity and inclination

Embedded class I objects are known to have outflows (e.g. Hogerheijde et al. 1998) so that an outflow cavity is included in our model envelope. Because of the presence of the outflow cavity, the inclination at which the object is observed is important, because at near-pole-on orientation one observes directly the central source and the disk. At larger inclinations, the envelope will be obscuring the disk.

A template model with the template PAH abundance and an envelope mass of 1.0

M, seen at varying inclination angles between 5 and 85◦is shown in Fig. 5.8. At an in-

clination ofi= 5◦the stellar radiation field (blackbody + scaled Draine field) is directly visible and no absorption features are present. Between inclination of 5◦ _{and 25}◦_{, i.e.,}

down the cavity and through the envelope, the appearance of the SED and the PAH features changes rapidly. The emission of the central star becomes obscured and the strength of the PAH features decreases by about a factor of 2. At increasing inclination the PAH features become weaker, but still dominate the spectrum. Ice absorption fea- tures can be seen at 3, 4.2, 6 and 15µm. At 85◦, the disk is observed almost edge-on and the PAH emitting regions are largely masked by the disk itself. The remaining features in the spectrum extracted for an infinitely large aperture are due to scattered emission, originating from higher up in the disk atmosphere. Observed PAH feature

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Figure 5.8: Model SED (top) and blow-up of spectrum (bottom) for i = 5, 25, 45, 65 and 85◦(black, red, blue, purple, gray, respectively). Curves fori= 25and 65◦ were omitted from the SED plot for clarity. The inset shows a blow-up of the 3.3µmPAH feature.

strengths will depend on the pointing and orientation of the limited aperture slit on the embedded source.

PAH abundance

Models with the total PAH abundance (both ionized and neutral species) varying be- tween the template abundance, and factors 10, 20 and 100 lower, are shown in Fig. 5.9. The envelope mass in these models is 1.0M. Reducing the PAH abundance by a factor

10 has the straightforward result of reducing the PAH peak flux by a factor 3. The 3.3

µmfeature is notably weak in all models, and already at a factor of 10 lower abundance not anymore detectable. Decreasing the PAH abundance also leads to an increase in the UV emission and continuum emission at far-IR wavelengths. The strongest feature, 7.7µm, is visible down to an abundance of1×10−8relative to H, i.e., a value 50x lower than the ISM.